1. Field of the Invention
The present invention relates generally to pressure sensors and, more specifically, to a pressure sensor which comprises a sensor die and a buffer member which are attached together to form a combined structure, wherein a fluid port is provided through the buffer member at a location that is offset from the sensing portion of the sensor die.
2. Description of the Prior Art
Many different types of pressure sensors are known to those skilled in the art. One particular type of pressure sensing device comprises a silicon die that is etched to form a cavity in a surface of the die. The etched cavity does not extend entirely through the thickness of the die but, instead, a diaphragm portion is left remaining in the opposite surface of the silicon substrate. The diaphragm portion is sufficiently thin to flex in response to differences in pressure on its upper and lower surfaces. In a manner that is well known to those skilled in the art, stress sensitive components can be disposed on the upper surface of the diaphragm portion over the cavity that is formed in the opposite surface. When the diaphragm portion flexes in response to pressure exerted on it, the stress sensitive components provide a means for detecting the flexing of the diaphragm portion. It is common for piezoresistors to be used as the stress sensing components.
In order to achieve miniaturization, an electronic circuit can be disposed on a common surface of the pressure sensor die with the stress sensing components. In addition, electrically conductive pads can be deposited on the same surface of the pressure sensor die in order to facilitate the connection of the die to external devices. The pressure sensor die is commonly made of silicon. In addition, it can be attached to a silicon or glass substrate which provides stress isolation between the diaphragm portion of the die and external components, such as fluid conduits, which are typically used to conduct a flow of a fluid to the cavity portion of the die or to the portion above the diaphragm portion of the die. Silicon pressure transducers of this general type can be used to measure either absolute pressure or differential pressure.
In the manufacture of pressure sensor components, the structures used to package the silicon die and the fluid conduits can lead to the introduction of additional stresses. For example, if the thermal coefficient of expansion of the attached devices is different than the thermal coefficient of expansion of silicon, temperature changes can induce stresses in the structure. Many different types of packaging techniques have been developed to minimize the introduction of stress and the transfer of stress from external components to the diaphragm portion of the die and its attached stress responsive components.
U.S. Pat. No. 5,257,547, which issued to Boyer on Nov. 2, 1993, discloses an amplified pressure transducer which incorporates numerous stress reducing characteristics. The pressure sensor is mounted to a ceramic plate with a buffer plate therebetween in order to isolate the pressure sensor from stresses that could be transmitted through the ceramic plate. The ceramic plate is necessary for the purpose of supporting a plurality of electronic components which comprise an amplification and compensation circuit. The plate is separated from all parts of its housing except a minimal central surface on a support boss which provides the support for the pressure sensitive device.
U.S. Pat. No. 5,187,985, which issued to Nelson on Feb. 23, 1993, describes an amplified pressure transducer circuit which includes a bridge arrangement of piezoresistors that are connected to a diaphragm that is sensitive to a pressure being monitored. The output of the sensor bridge is provided to a preamplifier which is, in turn, associated with an attenuation network and compensation circuitry. The transducer incorporates a plurality of temperature sensitive resistors within the compensation circuit and these temperature sensitive resistors are produced at the same time and during the same manufacturing step as the piezoresistors of the bridge arrangement.
U.S. Pat. No. 5,184,107, which issued to Maurer on Feb. 2, 1993, discloses a piezoresistive pressure transducer with a conductive elastomeric seal. The transducer utilizes premolded elastomeric seals in which at least one seal is electrically conductive. A piezoresistive stress sensitive element in the form of a diaphragm of semiconductor material having a thickened rim is held at its rim between a pair of premolded elastomeric seals in a two piece housing. Electrical connections with external circuitry are made by conductive paths through one of the elastomeric seals which makes contact with electrical leads which pass through the housing wall.
U.S. Pat. application Ser. No. 08/028,499, which was filed by Maurer on Mar. 9, 1993 and assigned to the Assignee of the of the present application (M10-15191), describes a pressure sensor with improved heat dissipation characteristics which is provided with a means for efficiently removing heat from a circuit portion of a sensor die by providing an elastomeric member between a first surface of the sensor die and electrical leads. A thermally conductive, but electrically insulative, portion of the elastomeric member is disposed between the circuit portion of the sensor die and the leads and a means is provided for urging the first surface of the sensor die into thermal communicating contact with the thermally conductive portion of the elastomeric member. In addition, a selectively conductive portion of the elastomeric member is disposed between contact pads on the first surface of the sensor die and electrical leads encapsulated within a portion of the sensor housing. The elastomeric member is also provided with an opening formed therethrough and aligned with the diaphragm portion of the sensor die to permit the media to be in fluid communication with the diaphragm of the sensor die. The improved sensor die arrangement provided by the present invention can be adapted for use in either an absolute pressure sensing embodiment or a differential pressure sensing embodiment.
In view of the difficulties commonly encountered when silicon sensor dies are packaged in housing structures and attached to fluid conduits, it would be significantly beneficial if a means were provided to reduce the possibility of inducing stresses in the pressure sensor diaphragm portion and transferring stresses from the attached packaging and conduit components to the diaphragm portion.